As an example of a charging device for charging a battery to which an insulation resistance detection circuit is connected, a charger disclosed in JP2010-239845A has been known.
The charger is used to charge an in-vehicle battery using an alternating current source. The in-vehicle battery is mounted in an electric vehicle, and is insulated from a body of the vehicle.
An insulation monitoring unit is connected to the in-vehicle battery to detect an insulation resistance between the in-vehicle battery and the vehicle body. The insulation monitoring unit includes a coupling capacitor, an alternating-current voltage generator, and an alternating current detector. An end of the coupling capacitor is connected to a negative terminal of the in-vehicle battery. The other end of the coupling capacitor is connected to an end of the alternating-current voltage generator. An end of the alternating current detector is connected to the other end of the alternating-current voltage generator, and the other end of the alternating current detector is connected to the vehicle body.
The charger includes a charging circuit and an earth detector. The charging circuit converts an alternating current supplied from the alternating current source into a direct current. The charging circuit is provided with an alternating-to-direct current converter. An input terminal of the charging circuit is connected to the alternating current source, and an output terminal of the charging circuit is connected to the in-vehicle battery.
The earth detector detects an earth fault between the output terminal of the charging circuit and the earth. The earth detector includes two resistors and a current detector. An end of each of the resistors is coupled to the output terminal of the charging circuit. An end of the current detector is coupled to the other end of each of the resistors, and the other end of the current detector is connected to the earth.
When the in-vehicle battery is charged, the vehicle body is grounded to the earth. The charging circuit converts the alternating current supplied from the alternating current source into the direct current, and charges the in-vehicle battery with the direct current. The earth detector detects the earth fault between the output terminal of the charging circuit and the earth. Because the vehicle body is grounded to the earth, the earth detector also detects a leak current flowing between the in-vehicle battery and the vehicle body.
A terminal of the AC source is grounded to the earth as a reference. In a case where the input side and the output side of the charging circuit are not insulated, a terminal voltage of the in-vehicle battery relative to the earth is likely to change with time. As a result, the current flows from the vehicle body to the earth through the coupling capacitor of the insulation monitoring unit. With this, the earth detector operates to stop the charging of the in-vehicle battery. Therefore, the in-vehicle battery is not charged.